Broadband coaxial line to waveguide transition



March 26, 1957 zALEsKl 2,786,981

BROADBAND COAXIAL LINE T0 WAVEGUIDE TRANSITION Filed May 18, 1954 INVENTOR. JOHNE ZALE SK/ United States Patent BROADBAND COAXIAL LINE T0 WAVEGUIDE TRANSITION John F. Zaleski, Valhalla, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application May 18, 1954, Serial No. 430,549

2 Claims. (Cl. 333--33) This invention relates to microwave transitions for joining two kinds of transmission lines or guides, and particularly to broadband transitions which do not require the use of tuned elements.

The many forms of transitions for use with rectangular hollow microwave guides, round guides, coaxial transmission lines and other kinds of conductors for microwave energy fall into several classes. In one class a resonant stub is provided to tune the transition for exact match of the transition discontinuity at a single wavelength. Such transitions are narrow band. In other transitions the dimensions are chosen to minimize mismatch. In still others the design is such as to produce a deliberate mismatch over a selected band of wavelengths. An iris or a plurality of tuning stubs or posts is then employed to neutralize the mismatch over a relatively wide band of wavelengths.

All of these presently existing and conventional forms of microwave transitions are alike in that the reflections of the microwave energy impinging upon them, due both to impedance mismatch and to geometrical discontinuity, are matched out more or less imperfectly by added elements or by carefully dimensioning selected elements for best results over a specified narrow band of wavelengths. Design of existing forms of transitions has been largely empirical and laborious and has resulted in forms which, although highly efficient and non-reflective over specified bands of wavelengths are in all cases restricted to use over bands very much narrower than the bands of usefulness of the guides or transmission lines which they join.

The instant invention is applicable in principle to transitions between any two forms of microwave transmission line or guide and particularly including coaxial line, hollow rectangular or round guide, three-wire microwave transmission line, or two-plane microwave transmission line. This invention employs the novel principle of preventing generation of disturbances due to the geometrical discontinuity of a transition, so that such discontinuity need not be matched out, thus leaving only the impedance discontinuity to be matched, which is relatively easy to do. This principle is in sharp contrast to the former principle of permitting a geometric discontinuity to cause a disturbance, then attempting to neutralize the disturbance in some manner.

The instant invention thus provides a transition which is qualitatively diiferent from all preceding transitions, because instead of neutralizing disturbances caused by the geometrical discontinuity of a transition junction, the present invention prevents the disturbances from arising at all.

In one form of the invention one of the microwave guides or transmission lines of the transition is carried beyond the junction of the transition to form a stub guide or line. The stub is then interdicted for the useful lowest order field mode while permitting the entrance of higher order modes, which however are very rapidly attenuated. interdiction is eifectively accomplished in one form of the invention by a median longitudinal sep- Patented Mar. 26, .1957

turn positioned in the stub in the direction of the transverse electrical component of the lowest order mode field.

It has been found that, with the addition of simple impedance discontinuity matching components, such a transition shows little frequency selectivity and can be used with the microwave energy flow in either direction.

Further understanding of this invention may be secured from the detailed description and the accompanying drawings, in which:

Figure 1 illustrates a transition from coaxial line to rectangular guide containing a matching septum in accordance with the invention and employing a doorknob matching device.

Figure 2 illustrates a similar transition employing a doorknob" matching device with one-half severed and removed.

Referring now to Fig. 1, a rectangular hollow guide 11 is joined to a coaxial transmission line 12 to form a transition for the transfer of microwave energy from one to the other in either direction. The coaxial line is perpendicular to one of the broad faces 13 of the rectangular guide, the median line 14 of which intersects the coaxial line axis. The coaxial central conductor 16 is conventionally centered in its outer conductor 17 and secured thereto by the insulating bead 18. In order to couple the guide field into the line, or vice versa, and in order to match the dissimilar impedances of guide and line to each other, the coaxial central conductor 16 is enlarged at or beyond the point where it enters the rectangular guide. The conductive enlargement 19 is con ductively secured to the floor 21 of the guide and is approximately hemispherical in shape, although it may instead have other forms such as wineglass form. Such a matching and coupling device is sometimes termed a doorknob.

The rectangular guide 11 is continued beyond the junction in a short hollow rectangular stub 22. In a transition as so far described energy which, for example, is applied to guide 11 at its entrance 23 is not all coupled through the junction and out the coaxial line 12, but some tends to continue straight past the junction if the rectangular guide be continued past the junction as just described. In order to return this energy to the junction in such phase as to reinforce the applied energy it has been customary to insert a tunable plunger in the stub to convert it into a short-circuited stub, the plunger distance departing from an exact quarter wavelength by the amount required to match out the reactance of the geometrical discontinuity. Such a device, however, is very frequency sensitive. In the present invention the entire length of the stub 22 is denied to the dominant TEio mode, while the junction is not short-circuited as it would be if the short-circuiting plunger were placed at the center of the junction. These two apparently irreconcilable objectives are accomplished by the use of a conductive septum, fin, or partition 24 which extends from contact with the central coaxial conductor 16 along the median E-plane of stub 22 to its end 26. The length of stub 22 need not be great, one-quarter of one wavelength in guide being sufiicient. The stub end 26 may either be open or be closed with a short circuiting plate, as no appreciable amount of energy reaches that point. The septum 24 extends between the two broad faces of guide stub 22, and is conductively secured to both. Septum 24 is also conductively secured to the enlargement 19 and to the central conductor 16.

Behavior of this transition may be visualized by considering that microwave energy applied to the entrance 23 of guide 11 enters the coaxial line 12 without appreciable impedance discontinuity reflection over a relatively wide band. Geometrical discontinuity ordinarily manifests itself by setting up higher order modes and by causing reactive reflections. In this case any higher order modes are propagated down the stub 22 which, being partitioned, does not support the dominant mode. The :higher order modes are rapidly dissipated and .do not cause appreciable loss, neither does the geometrical discontinuity cause measurable reflections or standing waves. A similar situation may be visualized when the microwave energy is applied to the transition from the coaxial guide.

Since the (dominant mode cannot exist in the stub 22, obviously that .part of the doorknob therein has no function and may be eliminated. Such a modified structure is illustrated in Fig. 2. The doorknob enlargement 19 is cut off at aplane 27 through the coaxial axis perpendicular to the guide axis, and the septum 24 is made to extend between both broad faces of the guide from the coaxial central conductor to the end 26 of the stub 22. The operation of this embodiment is the same as that of the embodiment of Fig. 1.

What is claimed is:

1. A microwave transition comprising, a rectangular hollow waveguide section capable of supporting only the dominate mode of microwave energy therein, a coaxial line the outer conductor of which is orthogonally coupled to a broad face of said waveguide section intermediate the ends thereof and the inner conductor of which is terminated in an enlargement conductively connected to the opposite broad face of said waveguide section, the axis of said coaxial line intersecting the longitudinal axis of said Waveguide section, and a conductive septum originating at and conductively connected to said inner conductor, said septum projecting into one end of said waveguide section perpendicular to and conductively interconnecting the broad faces thereof along their medial lines.

2. A microwave transition in accordance with claim 1 in which the terminating enlargement of said inner cou duct-or consists of a solid bounded by a pair of mutually perpendicular plane faces and a convex surface, one of said plane faces lying in the plane of the broad face of said waveguide section remote from said coaxial line and the other plane face extending transversely of said waveguide section with said convex surface extending into said waveguide section in a direction opposite to said septum.

References Cited in the file of this patent UNITED STATES PATENTS 

